Biogenic VOC oxidation and organic aerosol formation in an urban nocturnal boundary layer: aircraft vertical profiles in Houston, TX

摘要

Organic compounds are a large component of aerosol mass, but organic aerosol(OA) sources remain poorly characterized. Recent model studies havesuggested nighttime oxidation of biogenic hydrocarbons as a potentiallylarge OA source, but analysis of field measurements to test thesepredictions is sparse. We present nighttime vertical profiles of nitrogenoxides, ozone, VOCs and aerosol composition measured during low approachesof the NOAA P-3 aircraft to airfields in Houston, TX. This region has largeemissions of both biogenic hydrocarbons and nitrogen oxides. The lattercategory serves as a source of the nitrate radical, NO₃, a key nighttimeoxidant. Biogenic VOCs (BVOC) and urban pollutants were concentrated withinthe nocturnal boundary layer (NBL), which varied in depth from 100–400 m.Despite concentrated NO_x at low altitude, ozone was never titrated tozero, resulting in rapid NO₃ radical production rates of 0.2–2.7 ppbv h?1 within the NBL. Monoterpenes and isoprene were frequently presentwithin the NBL and underwent rapid oxidation (up to 1 ppbv h?1),mainly by NO₃ and to a lesser extent O₃. Concurrent enhancement inorganic and nitrate aerosol on several profiles was consistent with primaryemissions and with secondary production from nighttime BVOC oxidation, withthe latter equivalent to or slightly larger than the former. Some profilesmay have been influenced by biomass burning sources as well, makingquantitative attribution of organic aerosol sources difficult. Ratios oforganic aerosol to CO within the NBL ranged from 14 to 38 μg m?3OA/ppmv CO. A box model simulation incorporating monoterpene emissions,oxidant formation rates and monoterpene SOA yields suggested overnight OAproduction of 0.5 to 9 μg m?3.